What Are the Planets in Order?
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What are the planets in order from the Sun? Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. There are eight, they divide neatly into two very different groups, and knowing why they divide that way explains most of what is interesting about the solar system.
The order, with the basics
The four inner planets are Mercury, Venus, Earth and Mars. They are small, rocky and close together. The four outer planets are Jupiter, Saturn, Uranus and Neptune, and they are enormous, made mostly of gas and ice, and spread far apart.
The dividing line is the asteroid belt, and the reason for the split goes back to how the solar system formed. Close to the young Sun it was too hot for ice to survive, so only rock and metal could condense. Further out, ice was available in vast quantities, letting planets grow far larger.
The rocky four
Mercury is the smallest, closest and has almost no atmosphere, so its surface swings between extreme heat and cold. Venus is nearly Earth's size but has a thick carbon dioxide atmosphere and a runaway greenhouse effect that makes it the hottest planet at around 460 degrees Celsius.
Earth is the only one known to have liquid water on its surface and life. Mars is about half Earth's diameter, cold and dry now, but with clear geological evidence that liquid water flowed there in the past.
The giants
Jupiter is more massive than everything else orbiting the Sun combined, with a storm system, the Great Red Spot, larger than Earth. Saturn is famous for rings made of countless ice and rock particles, and is so low in density that it would float in a sufficiently large ocean.
Uranus and Neptune are often called ice giants because they contain more water, ammonia and methane than Jupiter and Saturn. Uranus is tipped almost on its side, so it rolls around its orbit rather than spinning upright.
Handy comparisons
A few facts that stick:
- •A day on Venus is longer than its year
- •Mars has the tallest volcano in the solar system, Olympus Mons
- •Jupiter has more than 90 known moons
- •Neptune has the fastest winds recorded anywhere in the solar system
What happened to Pluto
Pluto was reclassified as a dwarf planet in 2006. The trigger was the discovery of other similar sized objects in the outer solar system, which meant either admitting many more planets or tightening the definition.
The definition adopted requires a planet to orbit the Sun, be round under its own gravity, and have cleared other objects from its orbital neighbourhood. Pluto meets the first two but shares its region with many other bodies, so it fails the third.
How the outer planets were found
The six inner planets through Saturn have been known since antiquity because they are visible without a telescope. Uranus was the first planet ever discovered, spotted by William Herschel in 1781.
Neptune was found differently and more impressively. Astronomers noticed that Uranus was not following its predicted orbit, calculated where an unseen planet would have to be to explain the discrepancy, and found Neptune within a degree of the predicted position in 1846. It remains one of the most striking confirmations of Newtonian gravity ever made.
The takeaway
The planets in order are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, split into small rocky worlds close in and giant gas and ice worlds further out, with the split explained by the temperature of the early solar system.